THIS INVENTION relates to a process for producing hydrocarbons from a synthesis gas, and to catalysts. It relates in particular to a method of treating a catalyst support to form a modified catalyst support, to a modified catalyst support thus formed, to a method of forming a catalyst from the modified catalyst support, to a catalyst thus obtained; to a process for producing hydrocarbons, and to hydrocarbons thus produced.
According to a first aspect of the invention, there is provided a method of treating a catalyst support, which method comprises introducing onto and/or into an untreated catalyst support which is partially soluble in an aqueous acid solution and/or a neutral aqueous solution, Si, Zr, Cu, Zn, Mn, Ba, Co, Ni and/or La as a modifying component which is capable, when present in and/or on the catalyst support, of suppressing the solubility of the catalyst support in the aqueous acid solution and/or the neutral aqueous solution, thereby to form a protected modified catalyst support which is less soluble or more inert in the aqueous acid solution and/or the neutral aqueous solution, than the untreated catalyst support.
The catalyst support may, in particular, be in particulate form. The modifying component is thus present, in the modified catalyst support particles, on the particle surfaces and/or in internal support frameworks of the particles, ie the modifying component is chemically bonded to the particle surfaces and/or to support frameworks of the particles. For example, the modifying component may be chemically bonded to OH (hydroxy groups) on the support surfaces or via the formation of spinel structures with the support.
In principle, any commercially available catalyst support which is partially soluble in an aqueous acid solution and/or in a neutral aqueous solution, can be used. Examples of untreated catalyst supports that can be used are alumina (Al2O3), titania (TiO2) and magnesia (MgO). When the catalyst support is alumina, any suitable alumina support can, in principle, be used. For example, the alumina support may be that obtainable under the trademark Puralox SCCa 5/150 from CONDEA Chemie GmbH. Puralox SCCa 5/150 (trademark) is a spray-dried alumina support. Similarly, when the catalyst support is titania, any suitable titania support can, in principle, be used. For example, the titania support may be that obtainable under the trademark Degussa P25.
The introduction of the modifying component onto and/or into the catalyst support may include contacting a precursor of the modifying component with the catalyst support, for example, by means of impregnation, precipitation or chemical vapour deposition. Such modifying component precursors include compounds, e.g. salts or alkoxides, containing the modifying component or elements, viz Si, Zr, Cu, Zn, Mn, Ba, Co, Ni, and/or La.
In one embodiment of the invention, the modifying component precursor may, in particular, be a silicon-based modifying component precursor, e.g. an organic silicon compound or agent, so that the modifying component is silicon (Si) The organic silicon compound may be tetra ethoxy silane (xe2x80x98TEOSxe2x80x99) or tetra methoxy silane (xe2x80x98TMOSxe2x80x99).
When a silicon-based modifying component precursor is used with an alumina catalyst support, it may then be used in a quantity such that the silicon level in the resultant protected modified catalyst support is at least 0.06 Si atoms per square nanometer of the untreated or fresh support, preferably at least 0.13 Si atoms per square nanometer of the fresh support, and more preferably at least 0.26 Si atoms per square nanometer of the fresh support.
The upper limit of the modifying component, e.g. silicon, in the protected modified catalyst support may be set by parameters such as the degree of acidity imparted to the support by the modifying component and/or the porosity of the protected modified catalyst support and/or by the average pore diameter of the protected modified catalyst support. Preferably, the average pore diameter of the protected modified catalyst support as hereinafter described is at least 12 nm, as disclosed in South African Patent No. 96/2759, which is hence incorporated herein by reference thereto. Additionally, if an objective is to obtain, from the protected modified catalyst support, a catalyst having a composition of 30 g Co/100 g Al2O3, the untreated Al2O3 catalyst support, and also the protected modified catalyst support, must have a pore volume of at least 0.43 ml/g, as discussed hereinafter with reference to Table 2. The upper limit of the modifying component, e.g. Si, in the protected modified catalyst support is thus to be selected in such a manner that the geometry, e.g. the average pore diameter and porosity, of the protected modified catalyst support is not detrimentally effected to an appreciable extent. If the support acidity is negatively influenced by the modifying component, e.g. as may be the case when silicon is used as the modifying component, then the upper limit of the modifying component in the protected modified support can, instead, be set by the modifying component level at which the support acidity becomes unacceptable.
Thus, when spray-dried Puralox SCCa 5/150 (trademark) alumina is used as the untreated or fresh catalyst support, sufficient silicon-based modifying component precursor is used such that the upper limit of silicon in the resultant protected modified catalyst support is 2.8 Si atoms/nm2 of fresh catalyst support, preferably 2.5 Si atoms/nm2 of fresh catalyst support.
Instead, when spray-dried Puralox SCCa 5/150 (trademark) alumina is used as the untreated catalyst support (ie having a surface area of ca 150 m2/g and a pore volume of ca 0.5 ml/g implying an average pore diameter of ca 13 nm), then the maximum level of silicon may be set in accordance with Table 1.
The organic silicon compound or agent may be dissolved in an impregnation solvent, which is typically an organic solvent capable of dissolving the silicon compound, such as ethanol, acetone or propanol. The catalyst support may be admixed with the resultant solution to form a treatment mixture. The treatment mixture may be maintained at an elevated temperature for a period of time to impregnate the modifying agent into and/or onto the catalyst support. The elevated temperature may be at or near the boiling point of the impregnation solvent. The impregnation may be effected at atmospheric pressure, and the period of time for which the impregnation is effected may be from 1 minute to 20 hours, preferably from 1 minute to 5 hours. The excess solvent or solution may then be removed, to obtain a modified catalyst support. The removal of the excess solvent or solution may be effected under a vacuum of 0.01 to 1 bar(a), more preferably 0.01 to 0.1 bar(a), and at temperature equal to the boiling point of the solvent, e.g. using known drier equipment, fitted with a mixing device, and of which the jacket temperature is thus higher than the solvent boiling point.
The method may include calcining the silicon-containing modified catalyst support, to obtain the protected modified catalyst support. The calcination of the modified catalyst support may be effected at a temperature from 100xc2x0 C. to 800xc2x0 C., preferably from 450xc2x0 C. to 550xc2x0 C., and for a period of from 1 minute to 12 hours, preferably from 1 hour to 4 hours.
Calcination after support modification is necessary to decompose organic groups and to obtain the protected modified support. An optimized calcination time can be obtained by infra-red analysis of the modified support after calcination.
In another embodiment of the invention, the modifying component precursor may be an inorganic cobalt compound so that the modifying component is cobalt (Co). The inorganic cobalt compound, when used, may be cobalt nitrate (Co(NO3)2).
The inorganic cobalt compound may be dissolved in an impregnation solvent, which is typically water or an organic solvent capable of dissolving the cobalt compound, such as ethanol, acetone or propanol. The catalyst support may be admixed with the resultant solution to form a treatment mixture. The treatment mixture may be maintained at an elevated temperature for a period of time to impregnate the modifying agent into and/or onto the catalyst support. The elevated temperature may be at or near the boiling point of the impregnation solvent. The impregnation may be effected at atmospheric pressure, and the period of time for which the impregnation is effected may be from 1 minute to 20 hours, preferably from 1 minute to 5 hours. The excess solvent or solution may then be removed, to obtain a modified catalyst support. The removal of the excess solvent or solution may be effected under a vacuum, preferably a vacuum of 0.01 to 1 bar(a), more preferably 0.01 to 0.1 bar(a), and at a temperature equal to the boiling point of the solvent, e.g. using known drier equipment, fitted with a mixing device, and of which the jacket temperature is thus higher than the solvent boiling point.
The method may then include calcining the cobalt-based modified catalyst support, to obtain the protected modified catalyst support. The calcination of the modified catalyst support may be effected at a temperature from 400xc2x0 C. to 900xc2x0 C., preferably from 600xc2x0 C. to 800xc2x0 C., and for a period of from 1 minute to 12 hours, preferably from 1 hour to 4 hours to ensure the formation of a relatively water insoluble CoAl spinel layer uniformly covering the total support surface area.
In yet another embodiment of the invention, the modifying component precursor may be an organic zirconium compound so that the modifying component is zirconium (Zr). The contacting of the precursor and the calcination of the modified catalyst support may then be effected in similar fashion to the contacting and calcination hereinbefore described for the cobalt modifying component.
The invention extends to a protected modified catalyst support, when obtained by the method as hereinbefore described.
According to a second aspect of the invention, there is provided a method of forming a catalyst, which method comprises mixing a protected modified catalyst support as hereinbefore described with an aqueous solution of an active catalyst component or its precursor, to form a slurry, and impregnating the protected modified catalyst support with the active catalyst component or its precursor, to form the catalyst.
The active catalyst component precursor may be cobalt nitrate (Co(NO3)2) so that the active catalyst component in and on the catalyst is cobalt. The support may, as hereinbefore described, be alumina.
The method of forming the catalyst may be in accordance with that described in South Africa Patent No. 96/2759 which is thus incorporated herein by reference. Thus, the mixing of protected modified catalyst support and the active catalyst component or its precursor aqueous solution, and the impregnating, may comprise subjecting a slurry of the catalyst support or carrier, water and the active catalyst component or its precursor to a sub-atmospheric pressure environment, drying the resultant impregnated carrier under a sub-atmospheric pressure environment, and calcining the dried impregnated carrier, to obtain the Fischer-Tropsch catalyst in unreduced form. The unreduced catalyst thus obtained may be washed, e.g. with water, as also described in ZA 96/2759, to remove unwanted contaminants.
However, the water washing described in ZA 96/2759 can be omitted, under certain conditions. For example, if the following two-stage cobalt slurry phase impregnation and calcination of the active catalyst component precursor, in which the carrier or support is alumina, is followed, then water washing of the resultant catalyst is not required.
Thus, in a first stage or step, if it is assumed that the BET pore volume of the alumina support is x ml/g, and that y kg of the support is to be impregnated, the following procedure will ensure proper impregnation:
(1.82xy)kg Co(NO3)2.6H2O is dissolved in distilled water, aiming for a final volume of  greater than xy, preferably 2xy, liter. This solution is added to a vacuum drier, and heated to a temperature between 60 and 95xc2x0 C. To this solution, the total inventory of y kg support material is added at atmospheric pressure whilst continuous mixing of the slurry is maintained, e.g. by means of an internal rotating screw in a conical type vacuum drier. With the gradual application of vacuum, under continuous mixing at a temperature between 60 and 95xc2x0 C., the loss on ignition (L.O.I.) content of the slurry is reduced (over 3 (or more) hours) from  greater than (136.4x)/(1+1.86x), preferably (236.4x)/(1+1.86x) mass % to the state of incipient wetness. Loss on ignition (L.O.I.) is defined as the mass % loss observed during complete calcination, ie complete decomposition to Co3O4/Al2O3. This gradual drying procedure ensures that the cobalt is quantitatively drawn into the pores of the Al2O3 support without the occurrence of localized saturation, which results in premature crystallization of cobalt nitrate.
At the state of incipient wetness (L.O.I. of (136.4x)/(1+1.86x)), maximum vacuum ( less than 20 kPa(a)) should be applied whilst ensuring that the bed temperature does not drop below 60xc2x0 C. under continuous mixing. Once the stage of incipient wetness has been reached, vacuum drying should preferably proceed in an uninterrupted fashion, ideally at the conditions:
 greater than 60xc2x0 C. (but not higher than 95xc2x0 C.), and a vacuum of  less than 20 kPa(a).
Vacuum drying under these specific conditions should be maintained until a L.O.I.  less than 90% of the L.O.I. value at incipient wetness has been reached.
Direct calcination of this dried material in a fluidized bed, or a rotary kiln, calciner at 200-300xc2x0 C. (ideally 250xc2x0 C.) is then preferably effected.
In a second stage or step, if it is assumed that the BET pore volume of the first stage calcined material is xxe2x80x2 ml/g, and that yxe2x80x2 kg of this material is to be impregnated for a second time, the following procedure will ensure proper impregnation:
A maximum of (1,82xxe2x80x2yxe2x80x2) kg Co(NO3)2.6H2O can be added during this second impregnation, but this may exceed the aimed for cobalt loading of the catalyst. Table 2 provides the correlation between the pore volume of the starting Al2O3 (ie x ml/g), and the maximum attainable cobalt loading to be associated with a two-step impregnation procedure:
If the objective is a final catalyst of composition 30 g Co/100 g Al2O3, the starting Al2O3 support must have a pore volume 0.43 ml/g. If, however, the pore volume is larger than 0.43 ml/g, the estimated amount of (1.82xxe2x80x2yxe2x80x2)kg Co(NO3)2.6H2O should be adjusted in order to ensure the desired catalyst composition. This amount of Co(NO3)2.6H2O is dissolved in distilled water aiming for a final volume of  greater than xxe2x80x2yxe2x80x2, preferably 2xxe2x80x2yxe2x80x2, liter. This solution is added to a vacuum drier, and heated to a temperature between 60 and 95xc2x0 C. To this solution, the final inventory of yxe2x80x2 kg of the first stage material is added at atmospheric pressure, whilst continuous mixing of the slurry is maintained, e.g. by means of an internal rotating screw in a conical type vacuum drier. With the gradual application of vacuum, under continuous mixing at a temperature between 60 and 95xc2x0 C., the L.O.I. content of the slurry is reduced (over 3 or more hours) to the state of incipient wetness. As stated hereinbefore, this gradual drying procedure ensures that the cobalt is quantitatively drawn into the pores of the support material without the occurrence of localized saturation, which results in premature crystallization of cobalt nitrate.
At the stage of incipient wetness, maximum vacuum ( less than 20 kPa(a)) should be applied whilst simultaneously ensuring that the bed temperature does not drop below 60xc2x0 C. under continuous mixing. Once the stage of incipient wetness has been reached, vacuum drying should proceed in an uninterrupted fashion, ideally at the conditions:
 greater than 60xc2x0 C. (but not higher than 95xc2x0 C.), and a vacuum of  less than 20 kPa(a)
Vacuum drying under these specific conditions should be maintained until a L.O.I.  less than 90% of the L.O.I. value at incipient wetness has been reached.
Direct calcination of this dried material in a fluidized bed, or a rotary kiln, calciner at 200-300xc2x0 C. (ideally 250xc2x0 C.) is then preferably effected.
During either, or both, of the two abovementioned slurry phase cobalt impregnation steps, a water soluble precursor salt of Pt or Pd may be added, as a dopant capable of enhancing the reducibility of the active component. The mass proportion of this dopant, when used, to cobalt may be between 0.01:100 and 0.3:100.
The invention extends also to a catalyst, when obtained by the method as hereinbefore described.
This catalyst is thus in unreduced form, and requires reduction or activation before it can be used. This may be effected by subjecting it to heat treatment under the influence of a reducing gas such as hydrogen.
According to a third aspect of the invention, there is provided a process for producing hydrocarbons, which includes contacting a synthesis gas comprising hydrogen (H2) and carbon monoxide (CO) at an elevated temperature between 180xc2x0 C. and 250xc2x0 C. and an elevated pressure between 10 and 40 bar with a catalyst as hereinbefore described, after activation or reduction thereof, to obtain hydrocarbons, by means of a slurry phase Fischer-Tropsch reaction of the hydrogen with the carbon monoxide.
The invention extends also to hydrocarbons, when produced by the process as hereinbefore described.
It is known that an alumina supported cobalt based slurry phase Fischer-Tropsch catalyst produces a wax product when used in a Fischer-Tropsch reaction of a synthesis gas, comprising hydrogen and carbon monoxide.
Such catalysts have hitherto preferably been produced by slurry impregnation of an alumina support using an aqueous cobalt nitrate precursor solution, of which the pH can vary between 1 and 6. The alumina support partially dissolves in aqueous acid, as well as neutral aqueous solutions. After dissolution the aluminium ions can, in the presence of cobalt ions, re-precipitate as hydrotalcite-like structures, e.g. Co6Al2CO3(OH)16.4H2O. These amorphous hydrotalcite-like structures are physically adsorbed and loosely bonded to the original alumina surface. The formation of irregular structures on the surfaces of supports present after impregnation of, respectively, alumina with an aqueous nickel nitrate solution, magnesia with an aqueous ruthenium chloride solution and titania with an aqueous platinum chloride solution is also found. This phenomenon is thus not limited to alumina (Al2O3), but can also be found when using alternative supports such as magnesia (MgO) and titania (TiO2).
A serious problem that can arise when such catalysts, which are thus prepared on untreated supports, are used, as observed during larger scale pilot plant synthesis runs, is the undesired high cobalt content of the wax product. Commercialisation of the slurry phase Fischer-Tropsch synthesis process, using the known untreated alumina supported cobalt catalyst, can result in the wax product containing more than 50 ppm cobalt, after filtration through a Whatmans 42 (trademark) filter paper (hereinafter referred to as xe2x80x9cthe filtered wax productxe2x80x9d or xe2x80x9cthe filtered waxxe2x80x9d). During slurry impregnation of an untreated alumina support, using an aqueous cobalt nitrate solution, cobalt nitrate will deposit on the loosely bonded hydrotalcite-like structures. These cobalt on loosely bonded hydrotalcite-like structures can dislodge during extended runs and contaminate the wax product with cobalt rich ultra fines. These fines, of submicron nature, exit the reactor in the waxy hydrocarbon product. Due to the high cost of cobalt, this is a highly undesirable problem which has thus been solved, or at least alleviated, with this invention. Said alumina support should thus be protected during aqueous slurry impregnation by improving the inertness of the alumina surface, to prevent formation of cobalt ultra fines during Fischer-Tropsch synthesis. This is achieved in the present invention.
The invention will now be described in more detail with reference to the following non-limiting examples and with reference to the drawings.